For chip-removing face machining of metallic blanks or workpieces, e.g., engine-block blanks, face mills are used, which have diameters in a range of 80 mm to 700 mm, and which are equipped with replaceable milling inserts in a number that varies from 10 to over 100. A since long used milling insert for such milling cutter tools has a triangular basic shape, and is indexable as well as invertable by including two opposite, planar and mutually parallel support surfaces, as well as two usually endless, peripheral edge lines, which are countersunk in relation to the support surfaces, each one of which includes six chip-removing main cutting edges (two along each triangle side) and six surface-wiping secondary edges or wiper edges (adjacent to the corners). During operation, only one pair of edges co-operates, viz. one main cutting edge and one secondary edge, which are indexed forward to a position in which the secondary edge is located in an imaginary plane perpendicular to the rotation axis of the mill, at the same time as the main cutting edge is located at an angle (i.e. setting angle) of 45° to the plane. When the milling inserts are used in one and the same mill, e.g., a clockwise rotatable mill, three pairs of edges may be utilized on one side of the milling insert and three pairs of edges on the opposite the side, i.e., in total six pairs of edges. Furthermore, economical users may be even utilize all twelve pairs of edges, namely if the milling insert is moved to a mill that rotates in an opposite direction of rotation (counter-clockwise). In the tool, the individual milling insert is mounted with negative axial and radial angles in order to ensure the requisite clearances.
Previously, engine blocks were made from grey cast iron and were face machined by face mills, the milling inserts of which were distinguished in that the individual, straight main cutting edge transformed into a likewise straight, surface-wiping secondary edge via a corner, which, even if the same was not perfectly sharp, still was distinct by having at most one diminutive radius. However, such milling inserts involved problems originating in the scale- or flake-like structure of the grey cast iron, and which manifested themselves in so-called edge breakouts. Damages of this type could arise when the rim of milling inserts along the periphery of the mill approached the concluding end edge of the work surface. In particular, if the flakes of the material structure were disadvantageously oriented, the remaining edge piece could be broken loose from the rest of the material. In many cases, such edge breakouts led to the blank having to be discarded.
The above-mentioned problems were solved by a modification of the geometry of the milling inserts. More precisely, the secondary edge was given an arc-shape instead of a straight shape, so far that the planar clearance surface adjacent to the main cutting edge was allowed to transform into a convex clearance surface having a comparatively small radius adjacent to the secondary edge. Such pairs of edges generated a chip that was essentially equally thick along the part separated by the main cutting edge, but that became successively thinner and thinner along the part separated by the arched secondary edge. By this modification, the component forces directed against the remaining edge piece of the blank were redistributed so that the resultant force acted more downwardly than laterally. In particular, if the lateral component forces are great in relation to the downward directed forces, such great stresses are applied to the edge piece that it is torn loose laterally, in particular when the scale structure is inappropriately oriented.
In this connection, round or arched cutting edges generally produce more heat than straight edges. The simple reason therefor is that a straight edge has a minimal length for a given cutting depth and a given setting angle, and therefore separates a chip having a minimal width, while an arched edge, for the same cutting depth and having the same setting angle, is longer and separates a wider chip.
Recently, a new iron material has been developed, which is denominated CGI (Compacted Graphite Iron) and which has gained popularity for, among other things, the manufacture of engine blocks. Contrary to the oriented scale or the flake structure of grey cast iron, the structure of CGI is indifferent or coral-like, i.e., it lacks certain orientation. However, attempts to face mill workpieces of CGI by the milling inserts having round or arched secondary edges, which have been successful for the grey cast iron, have not turned out well. Among other things, for reasons difficult to understand, burr formation has occurred adjacent to the edge line along the generated surface where the mill exits.